1. Field of the invention
The invention relates to an axial-flow gas turbine, essentially consisting of a multistage turbine which drives, inter alia, a compressor arranged on a common shaft,
in which the shaft part lying between turbine and compressor is a drum which is surrounded by a drum cover, and in which the annular passage formed between drum and drum cover assumes the function of conducting the cooling air tapped from the compressor to the front end of the turbine rotor and after this to its rotor-side cooling passages,
and for which a labyrinth seal sealing against the drum cover is arranged on the drum for sealing between the pressure levels at the outlet of the compressor and at the inlet of the cooling air into the turbine,
all the rotor-side cooling air for the turbine being tapped from the compressor in the area of the compressor outlet.
2. Discussion of Background
Gas turbines of this type are known. All the rotor-side cooling air is tapped from the collecting space between compressor and turbine; most of it flows directly via an accelerating cascade into the rotor cooling passages. Here, the accelerating cascade is as a rule located on the same radius as the rotor cooling passages at the front end of the turbine rotor. The smaller portion of cooling air, i.e. the air necessary for cooling the last compressor disk as well as the drum and first turbine disk, must be recooled in a cooler for exercising the cooling function before it can be passed free of spin into the annular passage. This solution results in a number of shortcomings.
Firstly, the cooling air, since it is tapped from the collecting space, does not have the highest possible and desired purity as demanded, in particular, by the fine cooling passages of the blades.
Secondly, a separate, costly apparatus is required for the recooling.
Furthermore, this smaller, recooled portion of air, as a result of the convective heating-up, is in turn considerably heated up on its way up to the inlet into the annular passage, as a result of which the cooling effect is reduced.
In addition, the feeding of the air free of spin causes an additional increase in the adiabatic wall temperature in the relevant areas.
Finally, the feeding of the cooling air free of spin into the annular passage also produces a high heat transfer coefficient .alpha. in the entire rotor area to which the cooling air is admitted, which, together with the increased cooling-air temperature mentioned, can cause high transient stresses.
In addition, extremely high .alpha.-coefficients with the known disadvantages result in the area of the drum labyrinth.
In these known gas turbines, a return flow, i.e. an inflow of recooled air from the annular passage into the main passage of the compressor behind its last moving row is deliberately accepted. It goes without saying that this measure results in a not inconsiderable disturbance of the main flow.
Owing to the fact that the inflow into the rotor cooling passages inevitably takes place with little spin, the rotor has to perform pump work, which further increases the cooling-air temperature.